Photoconductive target for cathode-ray devices



Oct. 6, 1953 R. R. GOODRICH PHOTOCONDUCTIVE TARGET FOR CATHODE-RAY DEVICES Filed June 1 1951 Mam,

I ATTORNEY Patented Oct. 6, 1953 PHOTOCONDUCTIVE TARGET FOR CATHODE-RAY DEVICES Robert R. Goodrich, Cranbury, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 1, 1951, Serial No. 229,429

3 Claims.

This invention relates to targets for electron discharge devices, and, more particularly, to improvements in photo-conductive targets for cathode ray camera tubes of the type utilizing antimony tri-sulfide as a photo-conductor.

Television camera tubes employing photo-conductive targets and known as Vidicons are described in an article beginning on page 70 of the May 1950 issue of Electronics Magazine, in copending U. S. patent application, Ser. No. 198,130 of S. V. Forgue, filed November 29, 1950, and in a U. S. patent application of S. V. Forgue and R. R. Goodrich, filed concurrently with this one.

A Vidicon camera tube consists of an electron gun and a target assembly contained in a glass envelope approximately six inches long and one inch in diameter. The electron gun is of the conventional type used in the image orthicon and other television pickup tubes. The target assembly comprises a film of light-transparent, electrically-conductive material on a glass supporting surface, with a layer of photo-conductive material deposited upon the electrically-conductive film. The target and the gun are so arranged within the envelope that the electron beam from the gun scans the photo-conductive surface of the target.

The photo-conductive material used for Vidicon targets is an electrical insulator in the dark, but becomes electrically-conductive under the influence of light. The conductivity is proportional to the amount of light affecting the material, and is limited to the immediate area under the influence of the light.

Vidicons may be operated at either high or low 0 velocity. That is, they may be operated with the target at a sufiiciently high voltage positive with respect to the cathode so that the electron beam strikes the target with enough force to drive secondary electrons from the photo-conductive material, thereby rendering it more positive. Or, they may have cathode and target at approximately the same potential so that the scanning electron beam deposits electrons on the target with a negligible amount of secondary emission, thereby making the target more negative.

In one type of high velocity operation, the electron beam scans the photo-conductive surface facing the electron gun at a velocity above first crossover (i. e. at a sufificiently high velocity so first crossover potential differs with different target materials and surface conditions. Two hundred volts between target and cathode has given satisfactory results with red antimony trisulfide targets.

The target potential is applied to the electrically-conductive film in contact with the opposite side of the photo-conductive layer from the one scanned by the electron beam. A collector potential, about ten volts positive with respect to the target potential, is applied to a metal screen or ring, which is termed the collector electrode and is located immediately in front of the scanned surface of the photo-conductive layer. When the high velocity beam scans this surface, it releases secondary electrons which are attracted to the collector electrode making the scanned surface more positive until it reaches the potential of the collector electrode and equilibrium is established between them. When the scanning electron beam has brought the scanned surface to collector potential, it makes that surface ten volts positive with respect to the conductive film on the other side of the photo-conductive layer.

When a light image is focused upon the target, the photo-conductive material becomes conductive in the areas where the light affects it. The effect of this conductivity is to cause a leakage of electrons from the conductive film, through the photo-conductor, to the scanned surface. The amount of leakage depends upon the intensity of the light incident upon each area; and, its effect is to make the area of the scanned surface affected by the light a volt or so more positive. When the scanning electron beam rescans an area from which the electron charge has leaked, it restores the ten-volt difference between the surfaces of the photo-conductor. This leakage and restoration causes an electron current fiow in a circuit between the light-transparent, electrically-conductive film in contact with the photoconductive target and the source of potential to which it is connected. Variations in the electron current through this circuit, as more or fewer electrons are needed to restore the difference in potential, become the signal output of the tube.

In low velocity operation, the electrically-conductive film is connected to a source of potential approximately ten volts positive with respect tothe cathode which is at ground potential. The electron beam scans the photo-conductive surface; and, by depositing electrons thereon at a velocity less than first crossover (i. e. where the ratio of secondary electrons leaving the surface when primaries strike it is less than unity), brings the bombarded surface to cathode potential and produces approximately a ten volt difference of potential across the target.

When light is focused upon an area of the target, it renders the photo-conductive material conductive in that particular area, and causes the corresponding portion of the scanned. surface of the photo-conductor to giveup electrons and come a volt-or so closer in potential-to the conductive film. The next time the electron beam scans this area it restores to cathode potential the area from which electrons have leaked under the light-induced conductivity. Thim return to cathode potential restores the ten volt'differnce" across the target and causes an' electroncurrent to the conductive film front the-source of-p'oten tial to which it is electrically connected. This electron current through an outputresistorprovides the signal output from the tube.

Some of the qualities of photo-conductivematerials which affect their usefulness as Vidicon taigets are'z sensitivity, resistivityinthe dark; lag; useful li-fe'; and current-to-lig-ht response.

Sensitivity has reference to the: ability of. the material to become conductive under: the'infiuence' of light-. It'is'meas'ured'in micro-amperes ofvideo current output per lumen-'oflighton-the target;

Resistivity" in thedark has reference tothat quality of photo conduc'tive material which enableseit' to store amelectrioal-charge: in a given sp'ot without I leakage from front to back surf acc -as-=1ong: as there' is no light on the target. Dependent upon the resistivity of the: target-nia-- terial is its dark current" which. is the current flowing? through the: material between energized electrodes when there is: no light onthe target.

Bylag ismeant' rapidity'o'f response of the target to c'hanges'in'lightg .i. e. the 'a'bility of the target to erase a signal in a given period-of time without showing a-shadow-oi trail of-light: 'I he problems arisin'g from a lagbecome acute when light coloredmoving" object is televised against a dark background;

Usefullife has-reference to the hours/of operation that -can be expected froma target, and its" ability to stand up under the: handling etc. involved in' manufacturing processes.-

Current -to li-ght' response indicates:- the range of changesin light intensity which can-be covcred-within given limits of current output.

The concurrently filed application of Forgue and'Goodri-ch, identified above; describes a meth od for evaporating red antimony tri sulfiedphoto-conductive" targetsonto alight-transparent; electrically conductive surface in a Vidicon type of television pickup tub'e. Targets" prepared: in accordance with this methoda re characterized by exceptionally good: sensitivity;- but for some purposes they would be more satisfactory if they hadalo'wer level of background signal,- thereby producing a more uniform black. The background'signal is proportional to'the dark current of the photo-conductor.

Accordingly, it is an object of the resent invention to provide an improved photo-sensitive target for electron discharge devices.

Another object is to provide an improved photo-conductive target for cathode ray tubes} and one characterized by its substantial freedom from dark current.

The above and other related objectsare ac complishedin accordance with the invention by a newphotO-conductive target material comprisin'g amixture of' red antimony tri-sulfide with approximately one to five per cent of antimony oxide.

The invention is explained in more detail by reference to the accompanying single sheet of drawings wherein:

Fig. 1 is a view in section of a cathode ray camera'tube using the photoconductive target of the invention;

Fig. 2 is a plan view of a photo-conductive target in accordance with the invention.

The camera tube shown in Fig. 1 comprises a glassfenvelope' H containing an electron gun assembly l3and a target assembly I5. The glass face plate: ll' of: the envelope II is coated on its inner surface a light-transparent electrically-conductive'fi l-m [9. This film may be constituted of a compound of tin, such as tin oxide ortin chloride; A metal ring 2! passes through thegla'ss wall of the envelope It to make electrical contact with the conductive film i9.

A photo-conductive layer 23. is coated upon the electrically-conductivefilm l5 Thisphotoconductivelayer in accordance with the inven tion, consists of a mixture of redanti'mony. tri sulfide and antimony oxide.

Immediately: in front of the photo conducti-ve coating l9, inn-redirection of the approaching electrombeam, is a collector electrode fi li wh-ich consists of a wire mesh screen connected to-a metal ring 2'! which also passes through the glass wall-of the envelope llin the' same manner as the ring 2 which isconnected to the electrically-conductive film I91 The operation-otthe tube is explained previously. in this. specification.

Fig. 2 shows a plan View of the target assembly I5 of' Fig. 1,.- f'ronithe direction of the approa'ch'ing' electron beam. rams View, the photo conductive layer 23' of' redantimon-y. tri sul'fi'de mixed with antimony oxide is shown coated upon the electrically-conductive1ight -ti ansparent filin it which in turn is supported by the glass face plate [1.

Various combinations of antimony oxide and red antimony. trisulfide can be used -to produce a photo-conductive target, but the inventor has found by experiment that when the antimony oxide exceeds more than approximately 5% volume of the red antimony tri-sulfide,..the sons'it'ivity'o'f the target falls oift'o a considerable extent.

In preparingtargets in accordance with the invention, the antimon yoxid'e and antimony trisulfide should be thoroughly mixed when-they form the photo-conductive layer of the-target assembly. This can be accomplished by mixing the ingredients and precipitating, spraying, settling" etc. the mixture onto a: supporting surface; or evaporating thematerial in a crucible after the marineho'f the concurrently filed application of R. R. Goodrich and S. Vi Forgue identified above.

Alternatively, the two materials can be fused together as a mixture of antimony oxide and antimonytrisulfide', or when the antimony sul fide is originally combined f'ror'riits-constituent elements. They would then be applied by one of the methods suggested" above. Or,- theingredicuts couldbe simultaneously evaporated ontoa supporting. surface.

The percentage of antimony oxide to be mixed with the antimony tri-sulfide doesnot-appear to be very critical within the approximatelimits of from 1 to 5% by volume. But-when the volume of antimony oxide exceeds by more than 5%- the volume-of antimony tri-sulfide,- the sensitivity? of the resulting photo-conductor, as mentioned previously, falls oiT.

The antimony tri-sulfide used in this mixture maybe either the black or the red variety. But red antimony tri-sulfide is preferred because of its higher sensitivity and other desirable qualities.

Photo-conductive targets prepared in the manner described above have demonstrated good sensitivity with high resistivity in the dark and consequently a low level of back-ground signal, giving a uniform black level in a television system.

What is claimed is:

1. A light sensitive target for an electron discharge device comprising a support member and a coating formed of a mixture of antimony trisulfide and antimony oxide on said support member, said antimony oxide being in an amount of 1% to 5% of the mixture.

2. A photo-conductive target for an electron discharge device comprising a transparent support member and a coating formed of a mixture of red antimony tri-sulfide and antimony oxide on said support member, said antimony oxide being in an amount of 1% to 5% of the mixture.

3. A cathode ray camera tube comprising an evacuated envelope containing an electron gun and a light-sensitive target assembly, said target assembly comprising a light-transparent supporting member, a light-transparent electrically-conductive film on said supporting member, and a coating comprising a mixture of red antimony tri-sulfide and antimony oxide on said conductive film said antimony oxide being in an amount of 1% to 5 of the mixture.

ROBERT R. GOODRICH.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,281,474 Cartwright et a1. Apr. 28, 1942 2,335,705 Smith Nov. 30, 1943 OTHER REFERENCES Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 9, Longmans, Green 8: Co., 1929, N. Y., pages 576-8, 521 and 424. 

3. A CATHODE RAY CAMERA TUBE COMPRISING AN EVACUATED ENVELOPE CONTAINING AN ELECTRON GUN AND A LIGHT-SENSITIVE TARGET ASSEMBLY, SAID TARGET ASSEMBLY COMPRISING A LIGHT-TRANSPARENT SUPPORTING MEMBER, A LIGHT-TRANSPARENT ELECTRICALLY-CONDUCTIVE FILM ON SAID SUPPORTING MEM- 